In a step for allowing oxidation reaction to take place directly on an Si substrate (wafer) which is formed in the course of semiconductor process, and from which different Si crystal planes are exposed, in a conventional oxidation technique, oxidation speed is different depending upon crystal plane. As a result, there is a problem that an oxide film having uneven film thickness is formed on a substrate, and characteristics are varied depending upon location on the substrate.
As a step in which different Si crystal planes are exposed from a substrate, there is element-isolation known as Shallow Trench Isolation (STI), and a forming step of a vertical type MOS transistor in which an Si substrate is embedded. By forming a groove on the Si substrate by dry etching, different surface orientations are exposed from a side surface and a bottom surface of the groove. In the STI step, Si3N4 is exposed from the substrate surface by the oxidation step, and it is required to bring oxidation speed on the Si3N4 closer to oxidation speed on the Si substrate, and to obtain the same oxide film thicknesses as close as possible.
As conventional oxidation methods, there are a dry oxidation method and a wet oxidation method. In the dry oxidation method, the pressure of atmosphere in a reaction chamber is set to normal pressure or vacuum pressure, and oxidation processing of a substrate is carried out in atmosphere in which oxygen partial pressure is adjusted by means of oxygen alone, or N2, Ar and the like. In the wet oxidation method, oxidation processing of a substrate is carried out utilizing moisture formed by mixing oxygen and hydrogen with each other in a front stage of a reaction chamber. As a method for forming moisture by mixing hydrogen and oxygen with each other, there are widely utilized a method in which temperature is increased to ignition temperature of hydrogen and oxygen or higher by resistance heating or lamp light-gathering heating to burn, and a method in which hydrogen and oxygen are allowed to react with each other by catalysis at ignition temperature or lower (see Japanese Patent Application Laid-open No. H11-204511).
According to the conventional oxidation method, oxidation speed of (110) plane having greater Si atom surface density becomes two times greater than that of (100) plane in a thin film oxidation region depending upon Si atom surface density of a surface of an Si substrate between surface orientations of different Si substrates, e.g., between the (100) plane and the (110) plane. Further, oxidation resistance is high on an Si3N4, the Si3N4 is used as a barrier layer against oxidation, and oxidation does not proceed almost at all.